updated 6/14/2006 12:23:15 PM ET 2006-06-14T16:23:15

A large geyser that hadn't erupted since 1998 surprised two hikers near the edge of Yellowstone National Park's Norris Geyser Basin with a roar and burst of steam. Lee Whittlesey and Betsy Watry heard the Ledge geyser before they saw it. "It was like a jet plane," Whittlesey said.

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The geyser erupted at full bore around 5 p.m. Saturday, sending a plume of steam about 100 feet high. "I've been in the park 30 years, and this was the first time I'd seen Ledge erupt," said Whittlesey, who is Yellowstone's historian. "Now I can check that one off."

Watry, who works for the Yellowstone Association, said they were shocked at the show that unfolded about a quarter-mile away.

"We just stood there stunned and watched it for a while," she said.

The eruption coincided with other unusual activity at Norris over the weekend, including the eruption of other sporadic geysers and changes in surface water. Henry Heasler, Yellowstone's lead geologist, said Norris appeared to be undergoing a "thermal disturbance" -- an infrequent and often sudden shift in activity.

A smaller disturbance occurred in February. There were no disturbances last year. Such disturbances result from subsurface activity that brings water closer to the surface.

"Imagine if there was a big kind of geyser burp under most of Norris," Heasler said.

Among other changes, the usually quiet Vixen geyser has been erupting, Pearl geyser's water has changed from clear to opalescent, and water elsewhere in the basin has turned murky.

The duration of such disturbances is difficult to predict. Heasler compared the phenomenon to the ringing of a bell. "Some bells quiet down very quickly, and others can ring for a long time," he said.

Scott Bryan, author of "The Geysers of Yellowstone," said Ledge was active in the early 1970s until a thermal disturbance in 1974. After that, eruptions were less frequent until 1979, when it quieted down completely.

The geyser came back to life in 1993, with eruptions roughly every nine to 14 days, and fell silent again in 1998.

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